The need for rare earth metals such as neodymium (Nd), dysprosium (Dy) and praseodymium (Pr), samarium (Sm), terbium (Tb), is increasing much faster than its production. In the U.S., the demand is expected to further increase as the drive towards clean energy and high-tech devices increase. Neodymium (Nd), dysprosium (Dy), terbium (Tb), samarium (Sm) and praseodymium (Pr) are critical for high performance magnets strategic for technologies such as wind turbine power generators, electric motors in hybrid cars, and in everyday consumer products like computers, mobile phones amongst many others. Dy and Tb are enabling elements alloyed with iron (Fe) in Terfenol-D alloy; developed in naval ordinance laboratory for magnetostrictive applications such as naval sonar systems.
Nd, Tb and Dy are among the elements which the U.S. Department of Energy classified as being at risk of supply disruption. Recovering the critical elements from waste rare earth-containing materials can be economically and technically viable approach for addressing the supply risk challenge. Via recycling natural resources can be conserved, energy can be saved and less toxic wastes can be dumped in landfills. It is also desirable that a recycling process be environmentally friendly in order to reduce the emission of noxious fumes.
Recycling rare earth elements in materials can be either by directly reusing the materials or via chemical recovery of the rare earth elements. The chemical recovery methods for rare earth elements (REs) generally involve pyro-metallurgical and hydrometallurgical approaches. In the pyro-metallurgical approach, the REs recovery rates are reduced by the slag phase due to the high affinity of the REs with oxygen. Pyrometallurgical approaches also generate large amounts of solid waste and can be energy-consuming, although rare earths can be recovered as metals, instead of oxides. Hydrometallurgical routes enable higher recovery rates of REs, especially as oxides or other non-metallic forms. Nevertheless most hydrometallurgical approaches still require large energy consumption (pre-calcination) and the use of large amounts of chemicals, especially strong mineral acids such as hydrochloric, sulfuric and nitric acids. High volumes of wastes contaminated by strong mineral acids present some environmental problems. Investment to contain both the mineral acids and their contaminated wastes add to the cost of the process. The use of acids in hydrometallurgical routes increases the complexity of recovering rare earth elements from magnets contained in e-waste devices, such as HDDs. This is because the magnets would need to be presorted or pre-concentrated before materials recovery can begin. As a rule, magnetic materials containing rare earth metals are part of the complex diversified end-of-life products and their pre-concentration or physical processing after shredding is a mandatory requirement. There is therefore the need for a cost-effective, environmentally friendly method for recycling rare earth metal-containing materials, such as permanent magnet materials and terfenol-D.